This article reviews pertinent mechanical characteristics of the human musculotendinous unit. Contractile and elastic elements of muscle are discussed, primarily in relationship to their length-tension and force-velocity properties. Evidence from a variety of recent studies of muscle function and dysfunction is presented. Clinical implications for physical therapy practice are discussed, and examples of pathological conditions are cited. A primary conclusion reached is that the degree and duration of muscle elongation are important factors in immobilization and important considerations in therapeutic programs.